Games like Flux Ascendant
Games like Flux Ascendant include Vampire Survivors, Halls of Torment, Death Must Die, Megabonk, and Skul: The Hero Slayer. We matched them by the tags players give Flux Ascendant on Steam (Action Roguelike, Action, Bullet Hell, PvE, and Roguelike): closest matches first, and among equally close ones the better-known games.

Vampire Survivors
Shared: Action Roguelike · Bullet Hell · 2D

Halls of Torment
Shared: Action Roguelike · Bullet Hell · Action

Death Must Die
Shared: Action Roguelike · Bullet Hell · Action

Megabonk
Shared: Action Roguelike · Roguelike · Bullet Hell

Skul: The Hero Slayer
Shared: Action · Action Roguelike · Roguelike

Enter the Gungeon
Shared: Action Roguelike · Bullet Hell · Roguelike

BALL x PIT
Shared: Action · Bullet Hell · Action Roguelike

Rogue Legacy
Shared: Action Roguelike · Action · Roguelike

The Binding of Isaac: Rebirth
Shared: Action Roguelike · Roguelike · Pixel Graphics

Boneraiser Minions
Shared: Action Roguelike · Bullet Hell · 2D

Nuclear Throne
Shared: Action Roguelike · Roguelike · Bullet Hell

Dead Cells
Shared: Action Roguelike · Roguelike · 2D

Seraph's Last Stand
Shared: Action Roguelike · Bullet Hell · 2D

Brotato
Shared: Action Roguelike · Bullet Hell · Roguelike

Dungreed
Shared: Action Roguelike · Action · Roguelike

Risk of Rain Returns
Shared: Action Roguelike · PvE · 2D

Soulstone Survivors
Shared: Action Roguelike · Bullet Hell · Roguelike

Rogue: Genesia
Shared: Action Roguelike · Bullet Hell · PvE

Noobs Want to Live
Shared: Bullet Hell · Action · Roguelike

Rabbit and Steel
Shared: Action Roguelike · Bullet Hell · Roguelike

Rocket Rats
Shared: Action Roguelike · Bullet Hell · Action

Tower of Babel: Survivors of Chaos
Shared: Action Roguelike · Bullet Hell · Action

Hades
Shared: Action Roguelike · Action · Roguelike

Risk of Rain (2013)
Shared: Action Roguelike · Action · Roguelike